The invention concerns sensors for monitoring a medium, comprising an electromagnetic radiation source and a detector for electromagnetic radiation wherein the medium is located in the beam path between the electromagnetic radiation source and the detector and the refraction changes upon change of the medium.
The publication DE 10 2007 010 805 B3 discloses a method and a device for determining the urea concentration of a solution. For this purpose, light is emitted at various incident angles onto a boundary surface between a denser medium and a less dense medium, i.e., the body and the solution. For this purpose, a boundary surface between the body and the solution must be present. The light is then partially reflected at the boundary surface, depending on the incident angle, wherein with increasing incident angle the proportion of light reflected at the boundary surface increases. The reflected radiation is then detected by an appropriately arranged spatially resolving radiation detector.
The publication DE 10 2008 056 559 A1 comprises a sensor arrangement for detection of a first liquid medium in a second liquid medium by means of reflection of an emitted light beam as well as a correlated receiver. For this purpose, two glass rod lenses encapsulated in a housing are arranged parallel to each other. The glass rod lenses have a different optical refractive index than the liquid media. Opposite the glass rod lenses a reflection surface is arranged that is connected to the housing.
It is disadvantageous that depositions and contaminations of the boundary surface or of the reflection surface can falsify the measured result.